1. Field of the Invention
The present invention relates to an image forming apparatus, more particularly to an electro-photography type image forming apparatus for forming images while charging an image carrier.
2. Description of the Related Art
An electro-photography type image forming apparatus is designed, as known generally, to uniformly charge a surface of a photoconductor drum as being a drum type electro-photographic receptor. Conventionally, however, the corona electrifying method, characterized by having a corona occurring when a high voltage is applied to a thin corona discharging wire act on the surface of the photoconductor drum, has been commonly employed method. Recently, however, the contact charging method, advantageous in terms of the low-pressure process, the low ozone generation, the low cost, etc is getting popular. This method is characterized, for example, by that the charge roller, as being a charging roller member, is made to come into contact with the surface of the photoconductor drum thereby to apply a voltage to the charge roller to electrify the photoconductor drum.
The voltage to be applied to the charge roller may be DC voltage alone, but the AC voltage may also be applied so that the discharge to the positive and the negative can be made alternately for uniformly charging. For instance, it is known that charging the member to be charged can be made evenly when, for example, an oscillating voltage, obtained by superposing the AC voltage with a DC voltage (DC offset bias), such AC voltage having a peak-to-peak voltage equal to or higher than a discharge starting threshold voltage (charging start voltage) available when an AC voltage is applied.
When a sinusoidal AC voltage is applied to the charge roller, there occurs a resistive load current to flow in a resistive load between the charge roller and the photoconductor drum, a capacitive load current to flow in a capacitive load between the charge roller and the photoconductor drum, and a discharging current to flow between the charge roller and the photoconductor drum. The sum of these currents will flow in the charge roller. It is empirically known that an amount of the discharging current should be kept equal to or greater than a predetermined amount in order to maintain the discharge stable.
FIG. 1 shows a characteristic of the current Ic flowing through the charge roller when the charging voltage Vc is applied to the charge roller. In this case, Vc on the x-axis represents a peak value of the AC voltage, while the charging current Ic on the y-axis represents an effective value of the alternating current.
Gradually increasing the amplitude of the charging voltage VC causes the charging current to flow. Where the charging voltage is equal to or lower than the predetermined voltage Vh, the amplitude of the AC voltage is substantially in proportion to the charging current. This is because the discharge current will not flow where the resistance load current and the capacitive load current are in proportion to the voltage amplitude and the voltage amplitude is relatively small. Then, as the applied voltage is raised further, the discharge starts at the predetermined voltage (Vh), and the charging current Ic to the voltage amplitude comes off proportionality relation to flow in a value larger by the value of the discharging current, Is. In order to obtain a stable charge, it is sufficient to set the charging voltage to a level at which the value of the discharging current Is becomes larger than the predetermined value.
However, there have occurred cases where the increase in the amount of the discharge to the photoconductor drum not only accelerates the deterioration thereof such as the damage to the surface of the photoconductor drum but also causes the formation of abnormal image owing to the effect of the high-temperature and high-humidity environment coupled with the products formed during the discharging. Thus, in order to obtain a stable charge as well as to resolve such problem, it is necessary to minimize the discharge to be generated on the positive side and on the negative side alternately by applying the minimum necessary voltage.
Actually, the relationship between the voltage applied to the photoconductor drum and the value of discharge is not always constant but varies with the thickness of the photosensitive layer or dielectric, the material of the charging member and the changing condition of the environment such as the condition of the air. In the low-temperature and low-humidity environment, the material become dry and the resistance thereof become hard to increase, and thus it becomes necessary to apply the perk-to-peak voltage equal to or higher than a certain level. When the charging operation is carried out in a high-temperature and high-humidity environment regardless of that the operating voltage is set to the minimum voltage suiting the charging operation for obtaining a uniform charge in a low-temperature and low-humidity environment, the materials are apt to become too humid to cause a fall of resistance and resulting excessive discharging. Then, such an increase in the amount of discharge can give rise to the problems such as the poor image forming, the fusion of the toner, the cracking on the surface or the shortening of the life of the photoconductor drum.
Besides, it is also known that the fault caused by the change in the level of discharge is resulted also from the causes such as the variation of the quality occurring during manufacturing process, the variation of the resistance value owing to the contamination, the variation of the electrostatic capacity with the laps of time, the variation of the characteristic of the high-voltage generator and so on, in addition to the previously mentioned cause resulting from the variation of the environmental condition.
In order to prevent the changes in the discharge level, “Discharging Current Control Method” has been proposed (Refer to Japanese Patent Application Laid-open No. 2001-201921). In this method, the AC voltage to be applied to the charge member is made variable; the AC values are sampled respectively by the current sampling means at least at two voltage levels, namely, a voltage level lower than the voltage Vh at which the discharge starts and another voltage level equal to or higher than the voltage Vh; the optimal voltage for the optimal level of discharge is calculated to determine the level of the AC voltage to be applied to the charging member.
In FIG. 1, those points indicated by the circles and the corresponding letters, A, B, C and D, represent the points at which (the voltages) are sampled. The characteristics of the charging AC voltage Vc within the range, wherein the discharging current will not occur, and the characteristic of the charging current Ic are measured by sampling (the voltages) at the voltage levels, A and B, which are lower than the voltage Vh, at which the discharge starts. Similarly, two points, C and D, are sampled to measure the characteristic of the applied AC charging voltage Vc and the characteristic of the charging current Ic, within the range where the discharging current will not occur. Since the difference in characteristic between the above-mentioned two voltages corresponds to the discharging current Is, the level of the charging AC voltage, required for obtaining the discharging current of predetermined level, is calculated on the basis of the relationship between the above-mentioned two characteristics, and the level of the charging AC voltage is controlled according to the result of such calculation, thereby controlling the variation of the magnitude of the discharge.
However, the conventional discharge control method is considered to have the problems as set forth below.
(1) The sampling error by the current sampling means, if occurs, adversely affects the accuracy to a considerable extent in controlling the discharging current.
As discussed previously, in the conventional discharging current control method, the discharging current is calculated on the bases of the two relationships namely, the relationship between the characteristic of the discharging AC voltage Vc, sampled at the points (points A and B in FIG. 1), lower than the discharge starting voltage Vh, and the characteristic of the discharging current Ic, and the relationship between the characteristic of the discharging AC voltage Vc, sampled at another point, lower than the discharge starting voltage Vh, and the characteristic of the discharging current Ic. However, the levels of the charging currents at the points A an B differ largely from the levels of the charging currents at the points C and D, and so the occurrence of the sampling error can cause a substantial error of the calculated discharging current. This has been a drawback to the optimal control of the discharging current.
(2) Another drawback to the conventional method is that the continuous printing operation can cause the variation of the charging current magnitude. When carrying out the printing operation in the continuous printing mode, the temperature around the photoconductor drum rises to cause the change in the relationship between the applied voltage to the charge roller and the discharging current and the resulting change in the value of the discharging current. This entails the problem such as the inability for optimal discharging current control. In order to overcome such a problem, it can be devised to stop the printing operation or a predetermined period at predetermined intervals during the printing operation in the continuous printing mode to let the charging AC voltage fall to a level below the discharge starting voltage Vh to sample the level of the alternating current thereby to enable the level of the discharging current to be reset to the optimal level. It has been found, however, that this method cannot be an effective solution, since this method entails the slowdown of the printing speed of the image forming apparatus.